The present invention refers to a method of operating a reactor of a nuclear plant in which the reactor comprises a reactor vessel enclosing a core having a plurality of fuel elements and a number of control rods, wherein each fuel element includes a plurality of elongated fuel rods, which each has an upper end and a lower end and includes a cladding and nuclear fuel in the form of fuel pellets enclosed in an inner space formed by the cladding, wherein the fuel pellets are arranged in the inner space to leave a free volume in the inner space, wherein the free volume comprises an upper plenum, containing no nuclear fuel and provided in the proximity of the upper end of the fuel rod, a lower plenum, containing no nuclear fuel and provided in the proximity of the lower end of the fuel rod, and a pellet-cladding gap between the fuel pellets and the cladding, wherein a reactor coolant, during operation of the reactor, is re-circulated as a coolant flow through the core in contact with the fuel rods and is added to the reactor via a feed-water conduit as feed-water having a normal feed-water temperature providing a sub-cooling of the reactor coolant, and wherein each of the control rods is displaceable a control rod distance to be inserted into and extracted from a respective position between respective fuel elements in the core,
The reactor is a light water reactor and more precisely a boiling water reactor, BWR, or a pressurized water reactor, PWR. A method according to the prior art of this technical field is disclosed in WO 2005/122183.
In such a reactor, each fuel rod comprises a cladding and nuclear fuel in the form of a stack of fuel pellets of substantially uranium dioxide. The fuel pellets do not fill the whole inner space but there is also a free volume in the inner space in which the fuel pellets are permitted to swell, i.e. through thermal and irradiation expansion. According to prior art, the free volume includes or is formed by a gap between the fuel pellets and the inner side of the cladding, and by an upper plenum. The free volume, i.e. the inner space that is not filled by fuel pellets, is filled with helium to improve heat transfer in operation and to facilitate defect detection at manufacturing. Each of the control rods is insertable to and extractable from a respective position between (BWR) or in (PWR) respective fuel elements in the core in order to influence the power of the reactor, i.e. to control the power of the reactor and/or to shut down the operation of the reactor.
During unfortunate circumstances, it may happen that a smaller defect arises on the cladding of the fuel rod, a so-called primary defect. Such a primary defect can arise through wear from a foreign object. A small wear defect normally does not result in any significant dissolving and washing out of uranium dioxide from the fuel pellets of the fuel rod. A small primary defect may, however, result in a secondary degradation and the development of a larger secondary defect.
When a primary defect has been established, there is a communication passage for the reactor coolant to the inner space of the fuel rod. This means that water and steam may penetrate the inner space of the fuel rod until the internal pressure in the fuel rod is the same as the system pressure of the reactor. During this process, the inner side of the cladding and the fuel pellets will oxidize while releasing hydrogen from the water molecules in the reactor coolant. This release of hydrogen leads to an environment with a high partial pressure of hydrogen at a distance from the primary defect; a phenomenon, which is called “oxygen starvation” or “steam starvation”. In such an environment, the inner side of the cladding is inclined to absorb hydrogen, so called hydriding, which is a basic material property of zirconium and zirconium-based alloys. This hydrogen absorption results in a locally very high hydrogen concentration in the cladding, which significantly deteriorates the mechanical properties of the cladding. The cladding then becomes brittle and this can due to self-induced stresses or due to external load, give rise to crack initiation, crack growth and the development of a secondary fuel defect.
During normal operation of the reactor at principally full power, a primary defect can, as appears from above, arise in a fuel rod. It can be assumed that the defect fuel rod has an average load of for instance 20 kW/m, a certain pellet-cladding-gap, for instance 5-20 μm, and an internal pressure of for instance 5-100 bars. The internal pressure in fuel rods of a BWR lies during operation in the lower region of the interval, whereas the internal pressure in fuel rods of a PWR during operation can lie in the upper region of the interval. When the primary defect arises, the pressure difference between the internal pressure of the fuel rod and the system pressure will disappear, i.e. the internal pressure of the fuel rod will be the same as the system pressure. The system pressure in a BWR is typically about 70 bars, whereas the system pressure in a PWR typically is about 150 bars. When a primary defect occurs, the fill gas, which normally consists substantially of helium and fission gases from the fuel pellets, will be transported towards both the ends of the fuel rod. Steam will be introduced until the internal pressure of the fuel rod equals the system pressure.
Before the fuel rod is taken into operation and the radiation is initiated, the fill gas of the fuel rod normally consists substantially of helium and the internal pressure of the fuel rod is at room temperature typically 1-40 bars. The internal pressure in fuel rods for a BWR typically lies in the lower region of the interval, whereas the internal pressure in fuel rods for a PWR normally lies in the upper region of the interval. During operation some fission gas is released from the pellets and mixed with the fill gas. The total pressure is then increased and may at end-of-life exceed the system pressure for some fuel rods. In case of a primary defect, the pressure equalizes also in these cases. It is of importance that the released fission gas also includes inert gases, e.g. He, Xe and Kr.
As mentioned above, the steam will, after the occurrence of a primary defect and the introduction of water, react with the cladding and the fuel pellets during release of hydrogen from the water molecules, which react with the cladding or the fuel pellets. This means that an area with a very high partial pressure of hydrogen can be obtained at a distance from the primary defect. It is thus likely that very soon after the occurrence of the primary defect an area with fill gas has been formed at each of the two ends of the fuel rod. The free volumes, which are present directly adjacent to the ends, may initially contain substantially pure hydrogen gas, mixed with inert gases but free from steam. Since the partial pressure of hydrogen is very high in these areas directly after the occurrence of the primary defect, the risk for secondary degradation is high. However, if the partial pressure of hydrogen decreases and the partial pressure of steam increases, the local massive hydrogen absorption, and thus the risk for local secondary degradation will be reduced. The hydrogen absorption can take place more homogeneously over the whole inner side of the cladding wall.
Steam has inferior heat transfer properties as compared to He. Consequently, the fuel pellets normally increases in temperature subsequent to the occurrence of a primary failure and the associated steam ingress. The thermal expansion that is connected to the increase of the temperature of the fuel pellets further reduces the pellet-cladding gap and reduces the gas communication within the rod. Oxidation of the cladding and the fuel pellets will have a similar restrictive effect by the formation of oxides with lower density and larger volume.
WO 2005/122183 discloses a method according to which the risk for a secondary degradation can be reduced. More specifically, WO 2005/122/183 discloses a method for operating a reactor of a nuclear plant in which the reactor encloses a core having a plurality of fuel elements and a number of control rods. Each fuel element includes a plurality of fuel rods, which each includes a cladding and nuclear fuel in the form of fuel pellets enclosed in an inner space formed by the cladding. Each of the control rods is insertable to and extractable from a respective position between respective fuel elements in the core in order to influence the power of the reactor. The method includes the following steps: operating the reactor at a normal power during a normal state; monitoring the reactor for detecting a defect on the cladding of any of the fuel rods; reducing the power of the reactor after detecting such a defect; operating the reactor during a particular state during a limited time period during which the reactor at least periodically is operated at the reduced power in relation to the normal power; and extracting said inserted control rods after said time period for continuing operation of the reactor at substantially the normal state.
Since the reactor according to WO 2005/122183, when a primary defect has been detected, is operated at a reduced power, the nuclear reaction in the fuel will decrease. The temperature in the fuel pellets will thus decrease, which reduces the thermal expansion of the fuel pellets. Consequently, the free volume and the communication paths in the inner space of the fuel rod increases. This means that even more steam may penetrate the inner space of the fuel rod for maintaining the pressure equalization between the inner space of the fuel rod and the system pressure. In addition, the reaction rates for the oxidation of the cladding and the fuel pellets, and for the hydriding of the cladding, will decrease when the reactor power is reduced and the fuel temperature decreases. Since the defect fuel rod, when the power is reduced, has a substantially lower fuel pellet temperature and a substantially larger free volume in the inner space, the gases, i.e. the fill gas, formed fission gases, hydrogen gas and steam, will be mixed through diffusion. Diffusion will take place also at higher pellet temperatures, but the oxidation and hydriding rates may then be so high that the diffusion will have no significant importance in comparison to the gas movements arising due to the pressure difference between the different parts of the fuel rod. Consequently, according to WO 2005/122183, the gas mixing via diffusion will be the dominating mechanism for significantly decreasing and distributing the consumption of oxygen and hydrogen in the fuel rod. During these conditions, a gas mixing is thus obtained in the inner space at the same time as the hydriding is relatively slow. When a proper mixture of hydrogen and water molecules has been obtained in the inner space of the fuel rod, the hydrogen absorption at a continuing operation will take place more homogeneously along the whole fuel rod. It is thus possible to avoid the creation of a zone of the cladding, which has significantly degraded mechanical properties as a consequence of a powerful local hydriding. The homogeneous hydrogen distribution makes the fuel rod significantly less sensitive to crack initiation, crack growth and the development of a secondary defect. Consequently, the limited time period, during which the reactor is operated, at least periodically, at a reduced power, leads to a significant increase of the probability that the reactor with the same set of fuel rods thereafter can be operated until the next scheduled normal revision outage without any additional shut downs for removing defect fuel and without requiring the introduction of control rods for locally reducing the power in the region of the core where the defect fuel rod is located.
U.S. Pat. No. 5,537,450 discloses a device for detecting whether there is a fuel defect. The device is arranged to detect fuel defects during operation of the reactor by conveying a part of the off-gases from the reactor via a gamma spectrograph that continuously measures the nuclide composition and the activity level in the off-gases. It is also known to localize a fuel defect by a method called “flux-tilting”, which means that the control rods are operated one by one so that the power is changed locally in the core at the same time as the activity level in the off-gases is measured. An increase of the activity level in the off-gases can be correlated to control rod movements in the proximity of the fuel defect. In such a way the fuel defect can be localized. This method is time-consuming and during the time when the localization takes place, the power of the reactor is reduced to between 60 and 80% of full power.
U.S. Pat. No. 6,298,108 discloses a fuel rod for a BWR. The fuel rod has an upper end and a lower end and includes a cladding and nuclear fuel in the form of fuel pellets enclosed in an inner space formed by the cladding. The fuel pellets are arranged in the inner space in such a way that an upper plenum, containing no nuclear fuel, is provided in the proximity of the upper end of the fuel rod, and a lower plenum, containing no nuclear fuel, is provided in the proximity of the lower end of the fuel rod. The axial length of the lower plenum is approximately 50% of the axial length of the upper plenum.